Giant Planets: Hydrogen and Helium

The outer Solar System contains two planetary
behemoths and two planets which are merely enormous by our
standards. Jupiter & Saturn have hundreds of times the Earth's
mass; both radiate more energy than they receive from the Sun, and
this outflowing energy powers dramatic activity in their interiors
and atmospheres. Uranus & Neptune, each about 15 times the
Earth's mass, are less active. All have satellite systems and rings
shaped by subtle dynamical effects over trillions of orbits.

Topics

Gas Giants & Water Worlds

Internal Structure

Satellite Systems

Planetary Rings

Reading

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A Closer Look 7.1: Comparative Data for the Major Worlds

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p. 154

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7.1

Jupiter

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p. 153-155

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7.1b

The Great Red Spot

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p. 156

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7.1c

Jupiter's Atmosphere

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p. 156-157

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7.1d

Jupiter's Interior

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p. 157

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7.1e

Jupiter's Magnetic Field

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p. 157

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7.1g

Jupiter's Amazing Satellites

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p. 158-162

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7.2

Saturn

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p. 162-163

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7.2a

Saturn's Rings

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p. 163-165

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7.2d

Saturn's Moon Titan

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p. 167-168

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A Closer Look 7.4: Saturn's Rings and Moons from Cassini

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p. 169

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7.3

Uranus

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p. 170-171

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7.3c

Uranus's Interior and Magnetic Field

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p. 172-173

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7.4

Neptune

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p. 173

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7.4a

Neptune's Atmosphere

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p. 173-175

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7.4b

Neptune Interior and Magnetic Field

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p. 175

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7.4d

Neptune's Moon Triton

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p. 176-178

Overview of the Solar System

The Solar System can be divided into two zones.
Bodies in the inner zone are composed mostly of rock and
metal, while those in the outer zone consist largely of gas
and ice.

Gas Giants & Water Worlds

The four planets of the outer solar system come in
two general types. Jupiter and Saturn are gas giants: huge
spheres of hydrogen and helium with only traces of other elements.
Uranus and Neptune are water worlds: somewhat smaller planets
with a larger fraction of heavier elements (eg., oxygen, carbon,
nitrogen) than their giant neighbors.

Internal Structure: Pressure Balance

The concept of pressure balance helps us
understand the structure of giant planets (and stars), as well
as the atmospheres of terrestrial planets such as Earth.

Anywhere in a planet (or star), the pressure is just
the weight per unit area of the material above. On Earth at
sea level, for example, the weight of a column of air
extending to the top of the atmosphere and
1 square inch on a side is 14.7 lbs;
thus normal atmospheric pressure is 14.7 psi
(psi = `pounds per square inch'). This much pressure
is called `one atmosphere' or 1 atm.

A column of water 34 ft high and
1 square inch on a side also weighs
14.7 lbs, so the total pressure 34 ft
below sea level is twice what it is at the surface.

The pressure at the center of Jupiter is about
4×107atm. How can a gas resist such
enormous pressure?

Gas Pressure

The gas in a planet (or star) behaves somewhat like a spring.
Consider a column of gas like the one shown here. If there's
no gravity the gas is distributed uniformly.

With gravity pulling downward, the gas at the bottom of the
column is compressed, while the gas at the top spreads
out.

The compressed gas at the bottom of the column pushes
upwards, resisting the weight of the gas above it. The
further up the column, the less weight above and the less
upward push is required.

A spring supporting its own weight does the
same thing, compressing more at the bottom and less at the
top.

If the upward push of the compressed gas exactly
balances the downward pull of gravity everywhere, the planet (or star)
is in pressure balance.

Internal Structure: Jupiter and Saturn

Both Jupiter and Saturn have outer envelopes of gas
-- mostly molecular hydrogen and helium, along with hydrogen
compounds like water, methane, and ammonia. At some depth the
pressure becomes so high that hydrogen becomes a metal
and a good conductor of electricity. At the center of each planet
there's probably a core of rock and ice, about 15 times the mass of
the Earth.

Both planets release large amounts of internal heat,
powering convection throughout their interiors.

Question 5.1

Both Jupiter and Saturn contain metallic hydrogen.
However, most of Jupiter's interior is metallic, while most
of Saturn's interior is not. Why?

Jupiter contains a larger fraction of hydrogen than Saturn.

Jupiter is colder than Saturn.

Jupiter's internal pressure is higher because of its greater
mass.

Saturn receives less heat from the Sun.

Internal Structure: Uranus and Neptune

Uranus and Neptune have outer envelopes of hydrogen,
helium, and methane; below are mantles of water in ice or liquid
form, along with substantial amounts of methane and ammonia. Their
central cores of rock and ice are small, about the mass of the
Earth.

Curiously, Neptune has a significant heat flow
from its interior, but Uranus does not. The reason for this
difference is not known.

Magnetic Fields

Further evidence that the outer solar system has two
types of planets comes from magnetic fields. Jupiter and Saturn
have well-aligned fields generated by convection and rotation in
metallic hydrogen deep within their interiors. Uranus and Neptune,
in contrast, have off-center fields, suggesting more localized
sources in the mantles of these planets.

Satellite Systems

As of last count, Jupiter has 63 satellites, Saturn
has 56, Uranus has 27, and Neptune has 13. These numbers will
increase as further observations are made. At this point, however,
all the large satellites have been discovered; the objects we
are now finding are typically only a few kilometers in size.

Satellites divide into two groups. Regular
satellites are relatively close to their planets, have circular or
nearly circular orbits, and travel in the same direction as their
planet spins. Irregular satellites are further from their
planets, sometimes have quite elliptical orbits, and may travel in
either direction.

Europa: Rafts of Ice?

Europa's icy crust has fragmented and refrozen many
times, creating the complex pattern seen here. Blue areas have been
covered with icy dust from a fairly recent impact crater; red areas
are due to mineral contaminants.

The almost complete absence of craters on Europa
(left) suggests a very young surface; liquid water may have filled
in many craters. Ganymede's surface (middle) has many similar
features, but the scattered craters implies an older terrain. The
large number of craters on Callisto (right) shows that the surface
is very old.

Galilean Satellites: Internal Structure

Internal structures of Io, Europa, Ganymede, and Callisto.
The first three are differentiated, with metallic cores (grey)
and rocky mantles (brown); Callisto, in contrast, is not.
Europa and Ganymede have shells of water or ice (blue).

Resonance. I

This animation shows an example of resonance. The
inner satellite makes two orbits in the same time the outer
satellite makes one, so this is called a 2:1 resonance.

Animation of a 2:1 resonance

Resonance. II

Because these orbits are in a 2:1 resonance,
the inner satellite gets a small tug from its outer neighbor
every other time around. As a result, its initially circular
orbit becomes elliptical with time.

The orbit will keep getting more and more elliptical,
unless some form of friction acts to limit the
orbit's ellipticity.

Resonance: Jupiter's Satellites

The three inner satellites of Jupiter are in a 4:2:1
resonance; Io makes 4 orbits in the same time that Europa
makes 2 and Ganymede completes 1.

As a result, Io and Europa are forced into elliptical
orbits; their distances from Jupiter vary with time. Tides
from Jupiter create friction which keeps their orbits
roughly circular and heats their interiors.

Animation of a 4:2:1 resonance

Titan: A Satellite of Saturn

Ring Systems

All four planets in the outer solar system have
rings of icy or rocky particles traveling in circular
orbits. Saturn's rings are evident in even a small telescope; the
rings around Jupiter, Uranus and Neptune were not discovered until
recently.

Saturn's Rings

Saturn's rings are only about 20 m thick. They
orbit exactly over Saturn's equator.

Closeup of the Rings

Ring Formation

Rings can form when a small satellite comes too close to a
large planet. Within a distance known as the Roche
limit - about 3 times the radius of the planet - a small
satellite's gravity can't hold it together, and it is torn
apart by tidal forces. The debris spread out and eventually
form a ring.

Animation of ring formation

Since all four planets in the outer solar system
have rings, it seems that tidal disruption of small satellites is
pretty common.

Ring Structure: the Role of Resonance

The largest gap in Saturn's rings is known as the Cassini
division; under good conditions it is visible in a small
telescope. This gap is created by a 2:1 resonance with the
satellite Mimas. Many other gaps in the rings are also
associated with satellites.